Long-term biases in geomagnetic K and aa indices

Annales Geophysicae



Analysis is made of the geomagnetic-activity aa index and its source K-index data from groups of ground-based observatories in Britain, and Australia, 1868.0–2009.0, solar cycles 11–23. The K data show persistent biases, especially for high (low) K-activity levels at British (Australian) observatories. From examination of multiple subsets of the K data we infer that the biases are not predominantly the result of changes in observatory location, localized induced magnetotelluric currents, changes in magnetometer technology, or the modernization of K-value estimation methods. Instead, the biases appear to be artifacts of the latitude-dependent scaling used to assign K values to particular local levels of geomagnetic activity. The biases are not effectively removed by weighting factors used to estimate aa. We show that long-term averages of the aa index, such as annual averages, are dominated by medium-level geomagnetic activity levels having K values of 3 and 4.

Publication type Article
Publication Subtype Journal Article
Title Long-term biases in geomagnetic K and aa indices
Series title Annales Geophysicae
DOI 10.5194/angeo-29-1365-2011
Volume 29
Issue 8
Year Published 2011
Language English
Publisher European Geosciences Union
Description 11 p.
First page 1365
Last page 1375
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